1. Field of the invention
The present invention relates to an ink jet recording method and apparatus for recording a high-quality color image on a common paper.
2. Related Background Art
In a recording apparatus such as a printer, copying machine, and facsimile device, an image is recorded on a recording medium in a sheet form, such as paper, a thin plastic sheet or the like, wherein the image is formed with dot patterns on the basis of image information.
There are several types of recording apparatus categorized on the basis of the recording method. These include ink jet, wire dot, thermal, and laser beam recording apparatus. In the ink jet recording apparatus, ink droplets are discharged toward a recording medium thereby forming an image thereon.
The ink jet recording apparatus can form a high-resolution image at a high speed. Another advantage of the ink jet recording apparatus is that it is of the non-impact type and thus it provides quiet operation. Furthermore, it is possible to easily form a color image with a plurality of color inks.
Thus, the ink jet recording apparatus, which, as described above, records an image by discharging an ink droplet according to image information via a discharge opening of an ink jet recording head toward a print medium, is advantageously used in printers, facsimile devices, copying machines, etc., especially for applications in which quiet operation is required.
In one type of ink jet recording apparatus, ink is locally heated by means of thermal energy generated by a heating element (heater) so that a bubble is generated in the ink thereby changing the pressure in a nozzle and thus discharging an ink droplet. This type of ink jet recording apparatus has an advantage that an ink droplet can be discharged at a high frequency, and thus is widely used as a recording unit in various devices.
In the ink jet recording method, the following requirements should be met.
(1) The ink does not spread after being attached to a recording medium on which an image is to be formed.
(2) The ink can be preserved without a change in quality or characteristics.
(3) The apparatus and the ink can provide high safety.
In the case of color recording, the following requirements should also be met.
(4) No mixture of color inks (bleeding) occurs between different colors disposed adjacent to each other, which can occur when ink droplets are not fixed.
(5) When a color is recorded all over a certain area, the color can be formed uniformly over the entire area.
(6) The ink discharging stability should be excellent (in particular, when a recording operation is stopped for a while, it is required that good stability should be obtained when a recording operation is re-started).
However, if conventional ink is used to form a color image on paper of the common type, bleeding occurs and it is difficult to obtain a high-quality image. One known method to avoid this problem is to use a special type of recording medium called "coated paper" a surface of which is coated with a material having high ability of absorbing ink.
In the case wherein a so-called common (a paper of common type) paper such as a copying paper or a bond paper is used, the recording operation should be performed in an intermittent manner so that an ink droplet is discharged after passage of a time long enough for a previous ink droplet to dry thereby suppressing the bleeding. However, this method has a disadvantage that the recording operation is slow.
One possible method to avoid the above problem of the bleeding without using additional means is to employ an ink which quickly penetrates into plain a common paper when attached to it. Paper is a porous material including a great number of capillaries and the penetration amount h of ink droplets attached to a paper can be represented by equation (1) shown below which is known as Lucas-Washburm's equation. EQU h.sup.2 =.gamma..multidot.r.multidot.cos .theta..multidot.t/2.eta.(1)
where .gamma. denotes the surface tension of the ink, .eta., the viscosity of the ink, r, the average radius of capillaries, .theta., the angle of contact between the ink and paper, and t, the time required for the ink to penetrate into the paper.
If the time required for an ink droplet to completely penetrate into a paper after attached to the paper is denoted t.sub.a and the penetration amount of ink is denoted by h.sub.a, then the following equation (2) is obtained. EQU t.sub.a =(2.eta./.gamma..multidot.r.multidot.cos .theta.)h.sub.a.sup.2(2)
Furthermore, if the dot area formed by one ink droplet attached to a paper is denoted by S, the effective number of capillaries of the paper per unit area is denoted by N, and the volume of one dot of ink is denoted by V, then EQU V=.pi.r.sup.2 h.sub.a NS (3)
From equations (2) and (3), the following equation can be obtained: EQU t.sub.a =2.eta.V.sup.2 /(.gamma..pi..sup.2 r.sup.2 N.sup.2 S.sup.2 cos .theta.)=k.multidot.(.eta./.gamma. cos .theta.)(V/S).sup.2(4)
where k is a constant depending on the paper used.
In equation (4), .eta./(.gamma. cos .theta.) is the term relating to ink (ink term) and (V/S).sup.2 is the term relating to a head (head term). To obtain a small ink penetration time, it is required that the above ink and head terms should be small enough.
To determine the conditions which can effectively reduce the ink term .eta./(.gamma. cos .theta.) in equation (4) and thus can improve color image quality recorded on paper of the common type, the inventors of the present invention have measured the critical surface tension which is defined as the surface tension obtained when the angle e of contact between the paper and the ink droplet meets the condition cos .theta.=1 or .theta.=0.degree..
The critical surface tension has been determined using the Zisman plot of the contact angle .theta. measured for various kinds of liquids having different surface tensions dropped on paper.
FIG. 1 is a Zisman plot representing the wettability of various liquids dropped on the surface of paper of the common type.
The liquids used in the measurement include purified water, glycerin, hexanetriol, and triethylene glycol. Copying paper and bond paper, which are of the type most widely used, were employed as a commom paper.
From the plot shown in FIG. 1, it can be seen that the critical surface tension for the common paper is less than 35 dyne/cm or less. Therefore, if ink having a surface tension less than the critical surface tension determined above is employed, the ink will have extremely high wettability to the common paper and penetrate quickly into the paper. As a result, high-quality image with no bleeding will be formed.
The head term (V/S).sup.2 in equation (4) may be reduced by reducing a thickness of ink attached to paper. The reduction in a thickness of the ink can be achieved by reducing the size of an ink droplet and also increasing the discharge velocity of the ink droplet so that the ink droplet expand to a greater extent when it collides with the surface of the paper.
FIGS. 2 to 6 illustrate a process of discharging an ink droplet by means of thermal energy according to a conventional ink jet recording method.
FIG. 2 illustrates an initial state in which the liquid path 50 is filled with ink 53. A current (electrical signal) in the form of a pulse is instantaneously flowed through a heater (for example, an electro-thermal transducer) 51 thereby quickly heating a part of the ink 53 near the heater 51. As a result, so-called film boiling occurs and a great number of nuclei of small bubbles are generated in the ink 53 on the heater 51. The generated bubbles combine together into a single bubble and the resultant bubble starts quick expansion (FIG. 3). The bubble 52 continues to expand further and the ink 53 in the liquid path 50 is pushed out in both directions, that is, toward the discharge opening 54 and toward the ink supply opening (FIG. 4). Thus, a part of the ink 53a is discharged out via the discharge opening 54 at the end of the liquid path. At the time immediately after the discharging, the ink 53a still has a connection to the discharge opening 54 via a tail (FIG. 5).
The tail of the discharged ink 53a is cut off by the surface tension of the ink itself and also by the force which puts back the meniscus of the ink 53 formed at the surface of the discharge opening 54 of the liquid path 50. Thus, an ink particle 55 is formed and ejects toward a recording medium (not shown). When the pulse current flowing through the heater has been shut off and thus the bubble 52 has disappeared, the meniscus is drawn toward the inside of the discharge opening 54, and ink is supplied from the ink supply opening. As a result, the meniscus returns to the initial state via damping motion and thus discharge of another ink droplet becomes ready (FIG. 6).
In the case of an on-demand type recording head using a piezoelectric element, an ink droplet is discharged in a similar manner and motion of meniscus also occurs in a similar manner.
To suppress the bleeding, as described earlier, it is required that the surface tension of ink should be nearly equal to or less than the critical surface tension associated with recording medium on which an image is to be formed, and thus the ink used should have a very low surface tension.
However, in the conventional recording method of the on-demand type, if ink having a low surface tension is used, a tail is generated when an ink is discharged, and the tail is easily cut and divided into a great number of droplets so-called "satellite" extremely smaller than the main droplet. This problem remaining to be solved will be described in further detail below with reference to FIGS. 7 and 8.
FIG. 7 illustrates the discharge of ink having a surface tension of 48 dyne/cm, and FIG. 8 illustrates the discharge of ink having a surface tension of 28 dyne/cm. As can be seen from these figures, in the case of the lower the surface tension of ink, greater the number of satellite 66, satellite splashes 67, and mist (not shown) are generated.
Another problem which occurs when such ink having a low surface tension is used in a conventional recording method is that the damping time of the meniscus becomes too long and it becomes difficult to perform stable discharge of ink droplets.
Furthermore, if such the ink having a low surface tension is employed, the periphery of the discharge opening is likely to be wetted with ink and a great number of splashes and mist are generated.